1. Field of the Invention
The present invention relates to a device and a method for loading a sheet-like medium without tension onto a drum, and to a device and a method for unloading a sheet-like medium from a drum. The drum may be a vacuum drum of an imagesetter for example, or a vacuum drum of an input scanning machine.
2. Background
In many external drum imagesetters or input image scanners, media to be exposed are loaded onto a vacuum drum by means of suction caps. These suction caps cause bulges in the medium to be exposed, and when the medium to be exposed is taken over by the vacuum of the vacuum drum, these bulges are fixed on the drum. Any pattern that has to be recorded will thus be written on a deformed medium. Once removed from the vacuum drum, the medium takes its original shape without deformation, and the pattern undergoes a deformation which is, though not big, visible and often annoying.
European patent application EP-A2-0 228 545 discloses a film applying and removing device in a drum type image recording scanning apparatus in which the tip end of a film is ascended so as to contact the surface of the drum. Vacuum suction means directly takes the film. This device has a disadvantage: if the film is not perfectly aligned with the drum, it will wrap around the drum at a slant.
Exposed media are often removed from a drum by using suction caps as well. This takes quite a lot of time, as the suction caps need to be moved from the exposed medium wrapped around the drum to an acceptance device such as an unload tray or a developing machine.
European patent EP-0 533 145 discloses an apparatus and a method for removing a sheet from a hollow drum member. The apparatus includes means for supplying a vacuum to the interior of the drum, and two sets of vacuum openings through the surface of the drum member. The sheet is arranged to overlay and close the first set of vacuum openings with the second set of openings extending substantially parallel with an edge of the sheet and comprising only a small portion of the total number of vacuum openings through the drum. An exit blade is disposed adjacent to the drum and has an edge proximate to the drum that extends substantially parallel to the edge of the sheet. The exit blade is disposed with respect to the drum so as to form an acute angle with the surface thereof with the proximate edge of the blade closely adjacent the apex of the angle and parallel with the edge of the sheet so that, when the drum is rotated to a sheet removal position, the first set of vacuum openings lies beneath the acute angle when the edge of the sheet is disposed on the opposite side of the apex.
The method comprises the steps of: rotating the drum to a sheet removal position where the first set of vacuum openings lie beneath the acute angle and the edge of the sheet is disposed on the opposite side of the apex, and drawing air through the second set of vacuum openings by the vacuum in the drum and creating an area of sufficiently high pressure air at the apex to cause air to be forced past the edge of the exit blade with sufficient force to lift the edge of the sheet from the surface of the drum.
This apparatus and method have the disadvantage that means need to be provided for blowing air through the second set of openings in order to remove the sheet from the drum.